1. Field of the Invention
This invention relates to a more efficient and less expensive process for biological wastewater treatment that uses the activated sludge process to remove contaminants, including phosphates.
2. Description of the Related Art
Previously, wastewater treatment plants commonly removed phosphates from contaminated wastewater by one of three methods: chemically, using a process known as coagulation or chemical precipitation; biologically, using the activated sludge process that requires tanks connected in series for each of the metabolic processes of bacteria; or by a combination of chemical precipitation and the activated sludge process. None of these methods as previously practiced is entirely satisfactory. Increasingly stringent standards for effluent water quality have created a steadily growing need for less costly, more efficient phosphate removal.
Chemical precipitation is expensive, requiring the addition to wastewater of costly chemical flocculating agents and disposal of the metal contaminated sludge. In chemical precipitation, the treatment plant adds flocculating agents such as iron or aluminum salts to the wastewater. These flocculating agents form an insoluble hydroxide floc, which is a feathery, highly absorbent substance that absorbs colloidal and suspended solids. The entire floc settles and the precipitated phosphate is removed as sludge. While extremely effective--a treatment plant can dispose of its purified water in surface waters--these costly chemical flocculants maintain plant operating costs at a high level, and disposing of large quantities of sludge contaminated with metals increases the expense.
The activated sludge process, while less expensive because it requires no chemical flocculating agents or disposal of metal contaminated sludge, nevertheless depends on a carefully controlled sequence of metabolic processes of bacteria that removes less phosphate than chemical precipitation. These distinct metabolic processes, the oxic or aerobic, the anoxic, and the anaerobic, have been confined to separate tanks. These separate tanks that are connected in series require corresponding recirculation and return systems for the mixed liquor, and separate aeration, monitoring, and control systems. An example is published in R. Boll, Biological Nitrogen and Phosphorus Removal in Wastewater Treatment Plants, 1987 Institute fur Stadtbauwesen der Technischen Universitat Braunschweig 42. All the papers in that issue disclose that the oxic, anoxic, and anaerobic reactions for biological removal of phosphorous must be carried out in separate reactors, tanks, or reactor portions connected in series and having corresponding mixed liquor recirculation and sludge return systems.
In the oxic tank, the bacteria use dissolved oxygen supplied through an aerator to oxidize and decompose organic substances. The energy released on oxidation provides for bacterial growth and absorption of orthophosphates from the water that the bacteria convert to polyphosphates. The bacteria accumulate these polyphosphates in the interior of the cell. Additionally, ammonium that is present oxidizes to form nitrates and nitrides, a form of chemically bound oxygen.
Next in sequence, the anoxic phase of the reaction takes place in a separate tank. No aerators supply dissolved oxygen to this tank, and no dissolved oxygen is present to decompose organic substances. Instead, by a process of microbiological respiration in the absence of dissolved oxygen, chemically bound oxygen is released from the nitrates and nitrides. The nitrates and nitrides are converted to nitrogen, and organic substances are oxidized.
In the next reaction in the sequence, the anaerobic phase, no aerators supply dissolved oxygen to the tank, and no oxygen should be present as a nitrate or nitride. Under these process conditions, the bacteria convert energy-rich polyphosphates from the interior of the cell into orthophosphates that dissolve in the water, releasing energy. Dissolved organic material of low molecular weight continuously enters the tank and is absorbed and accumulated by the bacteria.
Phosphates are removed from wastewater by this process because more orthophosphate is absorbed from the water and converted to polyphosphate in the oxic tank than is released in the anaerobic tank. Because the process is cyclic, the higher the orthophosphate released in the anaerobic reactor, the even greater absorption and conversion of orthophosphate in the oxic reactor. Consequently, increasing the amount of orthophosphate released in the anaerobic reactor increases the amount of phosphate removed from the water by the bacteria.
Achieving a high orthophosphate release in the anaerobic reactor requires that no oxygen be supplied so that no dissolved oxygen is available, and that no chemically bound oxygen be present in the form of nitrates or nitrides. The oxic phase depletes the dissolved oxygen, and the anoxic phase depletes chemically bound oxygen.
The following table presents the process conditions for the desired metabolism at each tank:
______________________________________ Phase Conditions Metabolism ______________________________________ Oxic Oxygen supply; Oxidation of organic dissolved oxygen substances. Oxidation present; Formation of ammonium to nitrates of nitrates and and nitrides. Increased nitrides. absorption of orthophosphate from the water. Conversion of orthophosphate to cell polyphosphate. Anoxic No oxygen supply; Denitrification, i.e., no dissolved oxygen reduction of nitrate to present; reduction of nitrogen. Oxidation of nitrates and nitrides. organic substances. Anaerobic No oxygen supply; Conversion of cell no dissolved oxygen; polyphosphate into no chemically bound orthophosphate. oxygen (as a nitrate Microbiological energy or nitride). production. Release of orthophosphate in the water. Accumulation of organic substances. ______________________________________
These systems having separate tanks have several disadvantages that increase their cost and reduce their efficiency:
1. Operating costs remain higher than necessary for systems using several tanks connected in series.
2. Higher pumping costs result, in most cases, from the larger hydraulic head required by cascaded tanks connected in series.
3. Complicated and expensive mixed liquor recirculation and sludge return systems must be supplied for individual reactors.
4. The several smaller tank volumes are designed for a constant incoming pollution load, and cannot be satisfactorily adjusted to a variety of incoming pollution loads. Organic pollutant load, hydraulic flow, and even our understanding of pollutants and their compositions change every hour, day, week, and season of the year.
5. Capital costs for monitoring and control equipment increase since each reactor requires separate adjustment for aeration and mixing.
A wastewater treatment system is desirable that can achieve purification of wastewater in only one tank for all of the oxic, anoxic, and anaerobic reactions. Such a system would be less complex and less costly than a cascaded series of tanks. A one tank system should require lower hydraulic head and consequently lower pumping costs. Complicated and expensive recirculation and sludge return systems could be eliminated for all but the one tank, thus reducing capital expenditures and maintenance costs. Furthermore, a large volume, single tank system could handle a wide variety of hydraulic flow and pollutant loads.